1. Field of the Invention
This invention relates in general to a method for manufacturing an optical disc, a disc-shape recording medium, on which at least one of the recording or the reproducing process is performed by using a laser beam. The invention relates to a method for manufacturing an optical disc, wherein the cover layer is formed by adhering a sheet made of transparent resin to a disc substrate. More specifically, the invention relates to a method for transporting a multi-layered disc, wherein in the process of making the optical disc, the multi-layered disc is formed by loading the thin film sheet to the disc substrate through a liquid adhesive.
2. Description of Related Art
Discs for recording or reproducing information by a laser beam, such as a CD-R (Compact Disc-Recordable), a CD (Compact Disc), a DVD (Digital Versatile Disc), or a DVD-R (Digital Versatile Disc-Recordable) etc., have become popular. However, recently, there is a need for storing a large amount of information, such as video information, on disc, and thereby a high-density disc is being studied. The recording density of such a disc is substantially determined by the spot size of the laser beam irradiated on the disc. Assuming that the wavelength of the laser beam is λ and the numerical aperture of the objective lens is NA, the spot size is proportional to λ/NA. Therefore, in order to increase the recording density of the optical disc, the wavelength of the laser beam has to be shortened and the NA of the objective lens has to be increased. On the contrary, because the coma aberration created due to the tilt of the optical disc is increased in proportion to the third power of the NA, the tilt margin of the optical disc due to high NA is very small and the laser beam becomes dim due to a little tilt, so that it cannot achieve a high recording density. To increase the recording density, as disclosed by the Japanese Laid Open H11-31337, the cover layer, which is the transmission layer of the laser beam, is very thin (for example, about 0.1 mm), but it has to suppress the coma aberration due to the tilt of the optical disc increasing as the NA increases.
In the manufacturing process for the above optical disc, for example, a liquid ultraviolet curing adhesive is supplied to a resin disc substrate loaded onto a turntable of a spin coater. A sheet, which is a thin and transparent film, is deposed on the disc substrate via the ultraviolet curing adhesive to form a multi-layered disc. The multi-layered disc is then rotated at a high speed by the turntable so that the adhesive between the disc substrate and the sheet extends and becomes a thin film. Afterwards, the multi-layered disc is transported from the turntable to an ultraviolet ray irradiation apparatus. In the ultraviolet ray irradiation apparatus, the ultraviolet rays irradiate the multi-layered disc so that the ultraviolet curing adhesive filled between the disc substrate and the sheet becomes hard. In this way, the sheet is adhered to the disc substrate by the ultraviolet curing adhesive to form a cover layer.
When forming the cover layer by the sheet and the adhesive, it is very difficult to adhere the sheet, at least to within about 0.1 mm of the inner circumference of the sheet, to the disc substrate without damaging the quality of the optical disc, especially the appearance quality. However, the ultraviolet curing adhesive between the disc substrate and the sheet can be comparatively and simply uniformed in thickness due to its extension to the outer circumference by the centrifugal force. Furthermore, the surplus adhesive among the ultraviolet curing adhesive extended to the outer circumference can be prevented from being adhered on the surface of the optical disc because the surplus adhesive is shaken off at the outer circumference by the centrifugal force.
In addition, the conventional transporting apparatus for the multi-layered disc, for example, comprises a transporting arm to move the multi-layered disc body in the up-and-down direction and the horizontal direction, and a sucking unit arranged in the front end of the transporting arm. The transporting apparatus, by the sucking unit, sucks the upper surface of the multi-layered disc, i.e., the surface of the sheet, and then makes the transporting arm raise, lower, and rotate so that the multi-layered disc is raised from the turntable of the spin coater and then transported to the ultraviolet ray irradiation apparatus.
However, in order to adhere at least the inner circumference of the sheet to the disc substrate, the ultraviolet curing adhesive is applied to a ring region corresponding to the inner circumference of the sheet on the disc substrate before adhering the sheet. Otherwise, the ultraviolet curing adhesive has to be pushed onto the disc substrate by loading of the sheet on the disc substrate, to spread at least over the inner circumference of the sheet. At this time, even if only a little of the ultraviolet curing adhesive is surplus, ultraviolet curing adhesive is overflowed from between the sheet and the disc substrate to the inner circumference of the sheet. The ultraviolet curing adhesive overflowed from between the sheet and the disc substrate will be spun to the surface of the optical disc during the high speed rotation of the turn table, and this causes radiative blots when curing by the ultraviolet ray irradiation, resulting in the degradation of the appearance quality. In addition, as the optical disc with its sheet whose inner circumference is not well adhered to the disc substrate is repeatedly loaded to an optical drive, the sheet might be exfoliated from the vicinity of the inner circumference because the sheet is very thin and low in strength. Additionally, the appearance is worse to the eyes.
In addition, when the multi-layered disc is moved from the turntable to the ultraviolet ray irradiation apparatus by the aforementioned transporting apparatus, as the surface of the sheet is chucked by the chucking unit to raise the multi-layered disc using the transporting arm, an attraction force to the sheet created by a negative pressure in the chucking unit, or a load etc created by the weight of the multi-layered disc itself begins to act. At this time, because the sheet on the disc substrate is very thin, for example below 0.1 mm, wrinkles or local exfoliations might occur in the vicinity of the chucked portions due to the chucking unit. Similar to this is if the sheet is incompletely adhered to the disc substrate, and these wrinkles and exfoliations create degradation of the appearance quality. Furthermore, as the exfoliations occur on the sheet, the air will enter between the disc substrate and the sheet, causing that the adhesion between the disc substrate and the sheet to be incomplete. The exfoliations on the sheet expand from locations where the air enters when the optical disc is in use, and additionally the transmission rate of the laser beam can vary, causing errors of recording and reproducing.